1. Building Java Programs
Chapter 16
Lecture 16-1: References and linked nodes
reading: 16.1

2. NP-complete is a complexity class
No known polynomial time (O(n), O(n5)…) solutions!
Solutions are, for example, O(2n) – ouch!

3. Collection efficiency
Complexity class of various operations on collections:
Could we build lists differently to optimize other operations?
Method
ArrayList
Stack
Queue
add (or push)
O(1)
add(index, value) -
indexOf
get
remove
set
size
Method
ArrayList
Stack
Queue
add (or push)
O(1)
add(index, value)
O(N) -
indexOf
get
remove
set
size

4. Recall: stacks and queues
stack: retrieves elements in reverse order as added
queue: retrieves elements in same order as added
push
pop, peek
top 3 2
bottom 1
front
back 1 2 3
remove, peek
add
queue
stack

5. Array vs. linked structure
All collections in this course use one of the following:
an array of all elements
examples: ArrayList, Stack, HashSet, HashMap
linked objects storing a value and references to other(s)
examples: LinkedList, TreeSet, TreeMap
First, we will learn how to create a linked list.
To understand linked lists, we must understand references. 42 -3 17 9
front 42 -3 17 9
null

6. Non-contiguous memory
Array
Spread in memory 42 -3 17 9 42 9 -3 17

7. Arrays vs. linked lists Array advantages Array disadvantages
Random access: can quickly retrieve any value
Array disadvantages
Adding/removing in middle is O(n)
Expanding requires creating a new array and copying elements
Linked list advantages
Adding/removing in middle is O(1)
Expanding is O(1) (just add a node)
Linked list disadvantages
Sequential access: can't directly retrieve any value

8. A swap method? Does the following swap method work? Why or why not?
public static void main(String[] args) {
int a = 7;
int b = 35;
// swap a with b
swap(a, b);
System.out.println(a + " " + b); }
public static void swap(int a, int b) {
int temp = a;
a = b;
b = temp;

9. Value semantics
value semantics: Behavior where values are copied when assigned to each other or passed as parameters.
When one primitive is assigned to another, its value is copied.
Modifying the value of one variable does not affect others.
int x = 5;
int y = x; // x = 5, y = 5
y = 17; // x = 5, y = 17
x = 8; // x = 8, y = 17

10. Reference semantics
reference semantics: Behavior where variables actually store the address of an object in memory.
When one reference variable is assigned to another, the object is not copied; both variables refer to the same object.
int[] a1 = {4, 5, 2, 12, 14, 14, 9};
int[] a2 = a1; // refers to same array as a1
a2[0] = 7;
System.out.println(a1[0]); // 7 a1
index 1 2 3 4 5 6
value 12 14 9
index 1 2 3 4 5 6
value 7 12 14 9 a2

11. References and objects
In Java, objects and arrays use reference semantics. Why?
efficiency. Copying large objects slows down a program.
sharing. It's useful to share an object's data among methods.
DrawingPanel panel1 = new DrawingPanel(80, 50);
DrawingPanel panel2 = panel1; // same window
panel2.setBackground(Color.CYAN);
panel1
panel2

12. References as fields
Objects can store references to other objects as fields.
Example: Homework 2 (HTML Validator)
HtmlValidator stores a reference to a Queue
the Queue stores many references to HtmlTag objects
each HtmlTag object stores a reference to its element String
private Queue<HtmlTag> tags;
...
HtmlValidator
back
...
front
Queue
private String element;
...
HtmlTag
private String element;
...
HtmlTag l m t h
String y d o b
String

13. Null references null : A value that does not refer to any object.
The elements of an array of objects are initialized to null.
String[] words = new String[5];
not the same as the empty string "" or the string "null"
Why does Java have null ? What is it used for?
index 1 2 3 4
value
null
words

14. Null references
Unset reference fields of an object are initialized to null.
public class Student {
String name;
int id; }
Student timmy = new Student();
name
null
timmy id

15. Things you can do w/ null
store null in a variable or an array element
String s = null;
words[2] = null;
print a null reference
System.out.println(timmy.name); // null
ask whether a variable or array element is null
if (timmy.name == null) { // true
pass null as a parameter to a method
some methods don't like null parameters and throw exceptions
return null from a method (often to indicate failure)
return null;

16. Dereferencing dereference: To access data or methods of an object.
Done with the dot notation, such as s.length()
When you use a . after an object variable, Java goes to the memory for that object and looks up the field/method requested.
Student timmy = new Student();
timmy.name = "Timmah";
String s = timmy.name.toUpperCase();
Student
String
'T'
'i'
'm'
'a'
'h'
name
null
timmy
public int indexOf(String s) {...}
public int length() {...}
public String toUpperCase() {...} id

17. Null pointer exception
It is illegal to dereference null (it causes an exception).
null does not refer to any object; it has no methods or data.
Student timmy = new Student();
String s = timmy.name.toUpperCase(); // ERROR
Output:
Exception in thread "main"
java.lang.NullPointerException
at Example.main(Example.java:8)
name
null
timmy id

18. References to same type
What would happen if we had a class that declared one of its own type as a field?
public class Strange {
private String name;
private Strange other; }
Will this compile?
If so, what is the behavior of the other field? What can it do?
If not, why not? What is the error and the reasoning behind it?

19. A list node class Each list node object stores:
public class ListNode {
int data;
ListNode next; }
Each list node object stores:
one piece of integer data
a reference to another list node
ListNodes can be "linked" into chains to store a list of values:
data
next 42
data
next -3
data
next 17
data
next 9
null

20. List node client example
public class ConstructList1 {
public static void main(String[] args) {
ListNode list = new ListNode();
list.data = 42;
list.next = new ListNode();
list.next.data = -3;
list.next.next = new ListNode();
list.next.next.data = 17;
list.next.next.next = null;
System.out.println(list.data + " " + list.next.data
+ " " + list.next.next.data); // }
data
next 42
data
next -3
data
next 17
null
list

21. List node w/ constructor
public class ListNode {
int data;
ListNode next;
public ListNode(int data) {
this.data = data;
this.next = null; }
public ListNode(int data, ListNode next) {
this.next = next;
Exercise: Modify the previous client to use these constructors.

22. Linked node problem 1 What set of statements turns this picture:
Into this?
data
next 10
data
next 20
list
data
next 10
data
next 20
data
next 30
list

23. References vs. objects variable = value; For the list at right:
a variable (left side of = ) is an arrow (the base of an arrow)
a value (right side of = ) is an object (a box; what an arrow points at)
For the list at right:
a.next = value; means to adjust where points
variable = a.next; means to make variable point at 2
data
next 10
data
next 20 a 1 1 2

24. Reassigning references
when you say:
a.next = b.next;
you are saying:
"Make variable a.next refer to the same value as b.next."
Or, "Make a.next point to the same place that b.next points."
data
next 10
data
next 20 a
data
next 30
data
next 40 b

25. Linked node problem 2 What set of statements turns this picture:
Into this?
data
next 10
data
next 20
list
data
next 30
data
next 10
data
next 20
list

26. Linked node problem 3 What set of statements turns this picture:
Into this?
data
next 10
data
next 20
list1
data
next 30
data
next 40
list2
data
next 10
data
next 20
data
next 30
list1
data
next 40
list2

27. Linked node problem 3 How many ListNode variables?
Which variables change? 2 3
data
next 10
data
next 20
list1 1 5 6
data
next 30
data
next 40
list2 4 3 5
data
next 10
data
next 20
data
next 30
list1
data
next 40
list2 4

28. Linked node problem 4 What set of statements turns this picture:
Into this?
data
next 10
data
next
990
list
...
data
next 10
data
next
990
data
next
1000
list
...